Month: November 2020

There is no test for COVID-19.  Instead, testing focuses on detection of SARS-CoV-2, the virus known to cause COVID-19 or the body’s response to infection.  Today we will discuss antibody tests, the blood tests used to detect past infections.

Part of a healthy immune response to infection by any virus is the formation of antibodies.  Antibodies are made in plasma cells, a special type of lymphocyte, in response to an assault by a foreign pathogen such as a virus.  The immune response is amazing.  Once plasma cells “learn” how to identify the virus, they crank out millions of these little molecules which coat the surface of the virus, flagging them as “enemy”, and directing other cells of the immune system to isolate and eliminate them.  Once the infection is cleared, the plasma cells “remember” the code for that virus so they can be quickly recruited to make more antibody if the same infection occurs in the future.  This memory eliminates the time-consuming “learning” step.  This simple process explains why most people only get chickenpox once and explains how vaccines can protect children from mumps and measles. 

The first antibodies produced after infection are known as IgM Antibodies.  These are large pentamers, basically 5 antibody units brought together like a snowflake.  IgM antibody production is replaced with IgG antibody production generally after about a week, but the time varies by individual and virus.  IgG antibodies are produced for weeks to years after infection, again depending on the individual and the virus.  Other antibodies are produced by the immune system, but IgM and IgG are most useful in viral serology.  These antibodies are found in the blood stream, so a simple blood test can detect a variety of antibodies, including antibodies to SARS-CoV-2.  It is thought that antibodies to SARS-CoV-2 is what makes convalescent plasma an effective treatment for severe COVID-19.

There are several facts to keep in mind when interpreting antibody test results.  First, antibody tests tell us about past infections, not active infections.  Second, the antibody form tells us whether the infection was recent or distant.  IgM antibodies are made first but go away, so detection of IgM antibodies means the infection was recent.  IgG antibodies are made last but stay around for a long time.  Detection of IgG without IgM means that the infection occurred in the more distant past, at least three weeks ago, and detection of both IgG and IgM means the infection occurred in the transition period.

Antibody tests use an immunoassay methodology, which basically uses antibodies to detect antibodies.  Many of these tests have good specificity and sensitivity, but they are not perfect.  Cross reactivity and interfering substances can cause false positive and false negative results in the best of circumstances.  But we are not in the best of circumstances.  All SARS-CoV-2 antibody tests are available in the U.S. by an emergency use agreement (EUA) with the FDA.  Rigorous studies demanded by the FDA approval process have not yet occurred.

So, if you are positive for SARS-CoV-2 antibody, will you always be?  Don’t know.  If you are positive for SARS-CoV-2 antibody are you immune to COVID?  Don’t know.  It is simply too soon to define the body’s usual response to infection and its implication for future infection.  

What can we learn from antibody tests?  If your antibody test is positive, you probably had a SARS-CoV-2 infection in the past, even if you do not remember being sick.  If you do not have symptoms, and you have not been around someone with COVID-19, then you are not likely to have a current infection.  Continue normal activities, but with vigilant precautions.  Wear a mask and social distance.  These precautions reduce the spread of the disease.

Elon Musk recently tweeted that he was tested four times in the same day.  Two tests were positive, and two tests were negative.  How can this be? 

Mr. Musk was tested with the BD Veritor Plus rapid antigen test for SARS-CoV-2.  BD Veritor is one of several SARS-CoV-2 antigen tests made available in the United States by an Emergency Use Authorization (EUA) with the FDA.  While PCR tests have some limitations, antigen tests are fraught with many more challenges that bring their results into question.  

First the good news.  Antigen tests are cheap, plentiful, and rapid, usually providing results in 15 minutes or less.  And, with one exception, all antigen tests can be performed in laboratories operating under a CLIA Certificate of Waiver, meaning that they can be performed by personnel with very little education in laboratory science.

So what’s the bad news?  I see three major problems with antigen tests: the test process, the lack of amplification, and poor sensitivity.

The antigen tests from different vendors all follow, with minor variations, the same process.  A sample is collected on a swab, usually from the back of the nose (nasopharynx), and the swab is placed in an incubation well on the test device.  A few drops of reagent containing antibodies against the target (in this case, the SARS-CoV-2 virus capsule) are added.  If the antibodies bind to the target, a signal is sent to the test system.  This signal is usually a color change.  The whole process is very much like a home urine pregnancy test.  

Well, what could go wrong with that?  Strong lines are easy to see, but what about faint lines?  Where is the cut-off between positive and negative?  Some vendors include a reader calibrated to take the guesswork out of reading results, but you begin to see the problem.  The test system introduces an element of subjectivity and operator technique that varies from tester to tester, and these variations impact test results.  Recall that most of these tests are intended to be performed by personnel with very little training in laboratory medicine.  

This process lacks the amplification step of PCR.  That means when the swab is scraped against the back of the nose, what rubs off is all that you have for the test.  If you happened to not scrap off enough virus in an infected person, too bad—the test will be negative.

And that brings me to the final problem with antigen tests: low sensitivity.  According to the FDA submissions by BD, the antigen test used on Mr. Musk has a sensitivity that may be as low as 67%.  What does that mean?  Sensitivity of 67% means that one out of every three infected patients tests negative with the system. That’s right: wrong answers one-third of the time! And that is in the best of circumstances, using data that the manufacturer chose to submit to the FDA. These tests are not always used in the best of circumstances; remember, the test is approved for use by personnel with very little education in laboratory science.

BD makes the following statement in its submission to the FDA: “Negative test results do not preclude infection and should not be used as the sole basis for treatment or other patient management decisions, including infection control decisions, particularly in the presence of clinical signs and symptoms consistent with COVID-19, or in those who have been in contact with the virus. It is recommended that these results be confirmed by a molecular testing method, if necessary, for patient management.” Mr. Musk’s experience is not surprising.

My colleague says she has a Magic 8–Ball on her desk that is a lot like an antigen test: it’s cheap, plentiful and rapid. And, she says, it gives about as many correct answers. I agree.

We want a test to tell us whether someone has COVID-19, but that test does not exist.  Instead, laboratory tests look for the presence of SARS-CoV-2, the virus known to cause COVID-19.  A positive test does not necessarily mean that someone has COVID-19 and a negative test does not always mean that someone does not have COVID-19.  How can this be?  And if this is true, what is all the fuss about testing?

Let me be clear.  Testing is the most important tool available to determine who has COVID-19 and who does not.  But it is not quite as simple as “positive” equals COVID-19 and “negative” does not.  At least not quite.  The test must be interpreted, and to do so, a few simple nuances must be considered.

The most reliable test uses PCR methodology, which includes both amplification and detection steps, making it highly sensitive and specific for the detection of virus.

A negative PCR test means that an individual is not currently infected and has not been recently.  But a negative PCR test cannot be used to predict the future.  In other words, a negative test last week does not prove someone is virus-free today.  Instead, a negative test simply tells us the last time a person was known to be negative for virus.  The significance of a negative test result diminishes rapidly as the collection time fades into the past.  

A positive PCR test means that the person tested is or has been infected by the virus.  If the individual is also symptomatic, we can say the person has COVID-19.  That patient is also infectious (i.e., can spread the virus to others), and should be considered infectious for at least ten days after the first positive test, or for 24-hours after symptoms resolve, whichever is longer.

Our best understanding is that a person does not need to have COVID-19 to spread the virus to others.  Most people test positive five to seven days after exposure, but they can infect others for ten days from time of exposure, longer in exceptional cases.  That is why a person is considered infectious and instructed to quarantine for at least ten days after the first positive test, even without symptoms.  A PCR test may continue to be positive for weeks to months after infection, making it impossible to know whether asymptomatic people with a first-time positive test are newly infected or were infected in the past.  Because we cannot know when the person became infected, the asymptomatic individual should be considered infectious for at least ten days after the first positive test.

Another consequence of the persistence of positive PCR tests after the infection has cleared is that there is no need to require a negative test to prove an individual is no longer infectious.  After the appropriate amount of time has passed since the first positive test and/or resolution of symptoms, an asymptomatic individual should be considered virus-free for at least three months. 

If you may have been exposed, when should you test?  A test for SARS-CoV-2 becomes positive 2 to 14 days after infection, with most patients turning positive five to seven days later.  Most authorities suggest testing no earlier than five days after possible exposure, unless you have symptoms earlier.  But if you have to wait five days, should you even test at all? Whether you test or not may be a decision that addresses your peace of mind more than anything else.  The most important thing you can do if you think you may have been exposed is to self-quarantine for two weeks.  That is how you keep others safe and stop the virus spread.